Liesegang Patterns Engineered by a Chemical Reaction Assisted by

Letter pubs.acs.org/Langmuir

Liesegang Patterns Engineered by a Chemical Reaction Assisted by Complex Formation Hideki Nabika,* Mami Sato, and Kei Unoura Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa, Yamagata 990-8560, Japan S Supporting Information *

ABSTRACT: Liesegang rings based on a chemical reaction, not a conventional precipitation reaction, have been developed by appropriate design of the nucleation dynamics in a system involving complex formation in a matrix. The periodic and concentric rings consisted of well-dispersed Ag nanoparticles with diameters of a few nanometers. The approach modeled here could be applied to form novel micropatterns out of inorganic salts, metal nanoparticles, organic nanocrystals, or polymeric fibers, and it could also offer a scaffold for novel models of a wide variety of reaction-diffusion phenomena in nature.

■

displacement reaction.21,22 Although a complex system involving oxidation−reduction and acid−base gradient field has also been reported, the deposited rings were metal oxides/ hydroxides.23 A methodological breakthrough that enables pattern formation via chemical reactions, e.g., redox, polymerization, and organic reactions, would open up opportunities for new applications of Liesegang patterns in materials chemistry and robust models of reaction−diffusion phenomena. Herein, we propose a novel Liesegang mechanism that can utilize chemical reactions without a concentration threshold involved in the initial step. As the model system, we chose the chemical reduction of Ag+ with citrate in a gelatin matrix because the silver nanoparticles produced can be easily identified by eye or an optical microscope, owing to their bright color due to surface plasmon resonance. Furthermore, their structure can also be characterized easily using a transmission electron microscope (TEM). The most important step in the present system is the formation of a complex between Ag+ ions and methionine groups in the gelatin matrix. Although the rate of the reduction reaction between Ag+ and citrate in gelatin still has no concentration threshold, the following nucleation (Ag0 + Ag+ → Ag2+ and further growth) and aggregation processes forming the Agn clusters and Ag nanoparticles take place only when the number of Ag+methionine complexes in the same gelatin microdomain exceeds a critical level.24 In contrast to conventional methods of forming Liesegang patterns, which require a concentration threshold in the initial chemical reaction process, the present system has no such threshold, and instead, the threshold for the second nucleation process acts as the nonlinear process. Thus, the only difference from conventional Liesegang systems is that

INTRODUCTION Since their discovery in 1896,1 Liesegang patterns, with their unique and intriguing periodic structures, have attracted the attention of chemists, physicists, biologists, and geologists for more than a century, especially because they are ubiquitous in nature. The patterns can be formed by two oppositely charged ions precipitating when their concentrations exceed a threshold characterized by their solubility constant, resulting in patterns consisting of a variety of sparingly soluble inorganic salts such as hydroxide salts,2−5 chromate and dichromate,6−9 phosphates,10,11 oxalate,12 and others.13−15 The variety of patternformation processes offers a basis for developing a robust model to unlock the mysteries of biological Liesegang phenomena.16−18 Furthermore, recent developments in microfabrication processes have made it possible to fabricate complicated microtemplates for functionally patterned Liesegang materials consisting of inorganic salts with the desired size, shape, and symmetry.19,20 For both functional material applications and biological modeling using Liesegang phenomena, a diversity of reaction processes and products is critical. However, there is a mechanistic limitation for the Liesegang process that restricts this diversity, namely, that the concentration threshold should be involved in the initial chemical reaction step. Since the rate law of a general chemical reaction has the form of rate = k[A]m[B]n... and the reaction proceeds without any concentration threshold, in principle, Liesegang patterns cannot be formed by chemical reactions such as redox reactions. Therefore, to produce Liesegang patterns, one should utilize a precipitation reaction of sparingly soluble salts with inherent concentration thresholds characterized by their solubility constants. This is why Liesegang patterns have historically been mechanistically limited to sparingly soluble inorganic salts, with quite limited exceptions such as a precipitation reaction between oppositely charged nanoparticles and a galvanic © 2014 American Chemical Society

Received: January 27, 2014 Revised: April 20, 2014 Published: April 25, 2014 5047

dx.doi.org/10.1021/la5003786 | Langmuir 2014, 30, 5047−5051

Langmuir

Letter

Figure 1. (a) Optical microscope images of gelatin after contact with an agarose stamp. (b) Spacing-law plot for the rings in (a). The error bars in (b) indicate a standard deviation of 1 + p in each ring.

(Figure 2a), which is consistent with the behavior of a conventional Liesegang system in which the gelatin concen-

the solubility threshold is replaced by a complex concentration threshold. However, this means that a precipitation reaction is no longer required, which will enable us to design Liesegang patterns based on chemical reactions other than the precipitation reaction. The experimental procedure is also the same as those for the production of Liesegang patterns reported so far in that it involves placing an agarose stamp containing Ag+ ions on a gelatin matrix doped with citrate.

■

RESULTS AND DISCUSSION Ag ions diffuse into the gelatin after contact with agarose, and then the chemical reaction between Ag+ ions and citrates results in the reduction of Ag+ ions to Ag0. Since bare Ag0 atoms are extremely unstable, reduced Ag0 atoms rapidly form Ag nuclei (∼2.5 nm in radius), and a subsequent nucleation−growth process occurs.25 These processes proceed from the region nearest the agarose, and after 30 min, they were clearly observed as a thick donut-shaped ring surrounding the agarose stamp whose brownish color was characteristic of plasmonic Ag nanoparticles (Figure 1a). The ring initially grew in width continuously with time, but after 90 min, brownish concentric rings began to appear. The formation of concentric rings lasted for several hours, and after 240 min, the sample showed periodic patterns resembling Liesegang rings. One simple protocol to evaluate whether an obtained ring pattern is a Liesegang pattern is to determine whether the rings follow the spacing law 1 + p = xn+1/xn as n is large, where xn is the location of the nth ring in which the spacing between consecutive rings becomes constant with distance from the interface.9 Figure 1b clearly shows that the obtained pattern satisfies the spacing law, strongly suggesting that the pattern was formed by a Lieseganglike mechanism. Similar behavior, i.e., the formation of periodic bands that satisfy the spacing law, was also observed in a onedimensional system in a test tube (Figure S1). The spacing coefficient p decreased with increasing gelatin concentration +

Figure 2. (a) Gelatin concentration dependence of p. (b) Patterns formed at various AgNO3 and citric acid concentrations: (●) multiple rings, (Δ) single ring, (×) no ring. The error bars in (a) indicate the standard deviation of 1 + p for results under the same conditions.

tration affects the diffusivities of ions in the gelatin medium.9,26 Furthermore, the pattern formation was found to be limited to specific silver and citrate concentrations (see Figure S2 for original images). When the AgNO3 concentration of the solution used to prepare Ag+-containing agarose stamp was less than 0.5 M, no rings were obtained at any citric acid 5048

dx.doi.org/10.1021/la5003786 | Langmuir 2014, 30, 5047−5051

Langmuir

Letter

characteristic of the surface plasmon resonance of Ag nanoparticles formed by the chemical reduction of Ag+ ions with citrate.27 Thus, the color of the rings and the TEM observations indicate that they were made up of Ag nanoparticles produced by the chemical reaction between Ag+ ions and citrate, not a salt between an anion and cation. These results imply that during and/or after the diffusion of Ag+ ions into gelatin two processes proceeded independently: (i) rapid Ag aggregate formation without spatial periodicity and (ii) slow Ag nanoparticle formation in a Liesegang-like pattern. When the chemical reduction of Ag+ ions takes place in gelatin, the formation of complexes between Ag+ ions and methionine should be taken into account.24 Some Ag+ ions diffusing into gelatin are bound at the methionine sites and form Ag+-methionine complexes whereas the other Ag+ ions can freely diffuse in the gelatin. Since the Ag+ ions bound to methionine are stabilized against chemical reduction, their reduction rate has been reported to be reduced by a factor of 3 compared to that obtained for freely diffusing Ag+.24 Thus, the free Ag+ was first reduced to Ag0 in the gelatin by citrate. Since free Ag0 atoms are highly unstable and the reduction process proceeds without a concentration threshold, they rapidly aggregate and grow into large precipitates that begin to appear anywhere in the gelatin matrix without spatial periodicity. After the rapid growth of these dark precipitates, brownish concentric rings were observed where the remaining Ag+ ions in the gelatin were in Ag+-methionine complexes. The chemical reduction from Ag+-methionine to Ag0methionine also proceeds with standard kinetics without any concentration threshold. Although Ag0 is unstable and can easily aggregate via clustering without a stabilizing agent, as mentioned above, stabilization by methionine suppresses the clustering process to some extent. However, when the number of Ag+-methionine and Ag0-methionine complexes inside the gelatin microdomain increases owing to a continuous supply of Ag+ ions from the agarose stamp, a rapid clustering process is ignited in which large Agn clusters form by consuming the neighboring Ag species.24 This process corresponds to the supersaturation model of the Liesegang phenomenon, which requires a concentration threshold to trigger the precipitation reaction.28 The consumption of neighboring Ag species creates a region in which the Ag concentration is greatly reduced. The clustering process proceeds again in the region next to this depleted region, since the number of Ag+-methionine complexes in the same gelatin microdomain exceeds a critical level faster than in the depleted region. This is the same mechanism by which depleted regions are formed in conventional Liesegang phenomena.29 As such, concentric rings consisting of Ag nanoparticles would be formed following a mechanism similar to that for Liesegang rings. The importance of the methionine groups in gelatin was confirmed by replacing the gelatin with agarose (Figure S4). In this system, only a homogeneous formation of Ag nanoparticles was observed, and the nanoparticle concentration was a simple function of distance from the Ag+ ion source. As a step toward the application of Liesegang patterns in chemically programmed bottom-up fabrication processes, the construction of complicated periodic patterns using microstamp arrays has been studied in several papers.6,8,19 To demonstrate the suitability of the present system for such periodic patterning approaches and to yield further evidence that the obtained pattern was a Liesegang phenomenon, we have confirmed that interference patterns quite similar to the

concentrations (Figure 2b). At higher AgNO3 and citric acid concentrations, patterns with either single or multiple rings appeared. Multiple rings were obtained only for limited concentration conditions in the present experimental system. Under the same temperature and other experimental conditions, the concentration at which multiple rings appeared was limited to the range shown in Figure 2b. Since our ring satisfied the spacing law and was found under limited concentration conditions, it can be reasonably suggested that the ring patterns arose from a Liesegang-like mechanism. Magnified optical microscope images clarified that the overall pattern-formation process consisted of two independent processes on different time scales (Figure 3a). The original

Figure 3. (a) Magnified optical microscope images of gelatin after contact with an agarose stamp containing Ag+ ions. The right-side images are enlarged pictures of the regions in the black squares. TEM images of regions (b) outside and (c) inside the brownish concentric rings. (d) Enlarged image of nanoparticles observed inside the ring.

gelatin (0 min) was homogeneous and had no optically detectable defects or contamination. During the growth of the inner ring near the agarose stamp (40 min), optically detectable dark precipitates appeared without spatial periodicity over a large area extending up to a few millimeters away from the edge of the inner ring. Then, outer concentric rings slowly began to form after 60−90 min. The dark precipitates were stable and coexisted with the concentric rings throughout the pattern evolution (see the image at 180 min). We conducted TEM observations of regions both outside the ring, which contain only optically detectable precipitates, and inside the brownish concentric rings. The outside regions contained large aggregates ranging from a few hundred nanometers to a few micrometers in diameter, whereas well-dispersed nanoparticles were formed inside the rings. High-magnification TEM images (Figure 3d) revealed that the observed nanoparticles had a lattice spacing close to that of the Ag(111) plane (0.236 nm). Furthermore, in situ optical absorption measurements (Figure S3) of a ring region exhibited a progressive increase in the absorbance, especially below 550 nm toward 400 nm, which is 5049

dx.doi.org/10.1021/la5003786 | Langmuir 2014, 30, 5047−5051

Langmuir

Letter

(4) Badr, L.; El-Rassy, H.; El-Joubeily, S.; Sultan, R. Morphology of a 2D Mg/NH4OH Liesegang pattern in zero, positive and negative radial electric field. Chem. Phys. Lett. 2010, 492, 35−39. (5) Al-Ghoul, M.; Ammar, M.; Al-Kaysi, R. O. Band Propagation, Scaling Laws and Phase Transition in a Precipitate System. I: Experimental Study. J. Phys. Chem. A 2012, 116, 4427−4437. (6) Bensemann, I. T.; Fialkowski, M.; Gryzybowski, B. A. Wet Stamping of Microscale Periodic Precipitation Patterns. J. Phys. Chem. B 2005, 109, 2774−2778. (7) Karam, T.; El-Rassy, H.; Sultan, R. Mechanism of Revert Spacing in a PbCrO4 Liesegang System. J. Phys. Chem. A 2011, 115, 2994− 2998. (8) Smoukov, S. K.; Lagzi, I.; Grzybowski, B. A. Independence of Primary and Secondary Structures in Periodic Precipitation Patterns. J. Phys. Chem. Lett. 2011, 2, 345−349. (9) Lagzi, I. Controlling and Engineering Precipitation Patterns. Langmuir 2012, 28, 3350−3354. (10) George, J.; Varghese, G. Intermediate Colloidal Formation and the Varying Width of Periodic Precipitation Bands in Reaction− Diffusion Systems. J. Colloid Interface Sci. 2005, 282, 379−402. (11) Karam, T.; El-Rassy, H.; Zaknoun, F.; Moussa, Z.; Sultan, R. Liesegang banding and multiple precipitate formation in cobalt phosphate systems. Chem. Phys. Lett. 2012, 525−526, 54−59. (12) Xie, A.; Zhang, L.; Zhu, J.; Shen, Y.; Xu, A.; Zhu, J.; Li, C.; Chen, L.; Yang, L. Formation of calcium oxalate concentric precipitate rings in two-dimensional agar gel systems containing Ca2+−RE3+ (RE = Er, Gd and La)−C2O42−. Colloids Surf., A 2009, 332, 192−199. (13) Mandalian, L.; Fahs, M.; Al-Ghoul, M.; Sultan, R. Morphology, Particle Size Distribution, and Composition in One- and Two-Salt Metal Oxinate Liesegang Patterns. J. Phys. Chem. B 2004, 108, 1507− 1514. (14) Al-Ghoul, M.; Ghaddar, T.; Moukalled, T. Pulse-Front Propagation and Interaction During the Growth of CdS Nanoparticles in a Gel. J. Phys. Chem. B 2009, 113, 11594−11603. (15) Tomikawa, N.; Nanzai, B.; Igawa, M. Preliminary Study of Quantitative Analysis of Ammonium Ions in a Raindrop Following Liesegang Ring Formation. Anal. Sci. 2011, 27, 861−864. (16) McGuigan, H.; Brough, G. A. Rhythmic Banding Of Precipitates (Liesegang’s Rings). J. Biol. Chem. 1923, 58, 415−423. (17) Hein, I. Liesegang phenomena in fungi. Am. J. Bot. 1930, 17, 143−151. (18) Tuur, S. M.; Nelson, A. M.; Gibson, D. W.; Neafie, R. C.; Johnson, F. B.; Frank, B.; Mostofi, F. K.; Connor, D. H. Liesegang rings in tissue. How to Distinguish Liesegang Rings from the Giant Kidney Worm, Dioctophyma Renale. Am. J. Pathol. 1987, 11, 598− 605. (19) Klajn, R.; Fialkowski, M.; Bensemann, I. T.; Bitner, A.; Campbell, C. J.; Bishop, K.; Smoukov, S.; Grzybowski, B. A. Multicolour Micropatterning of Thin Films of Dry Gels. Nat. Mater. 2004, 3, 729−735. (20) Grzybowski, B. A.; Bishop, K. J. M.; Campbell, C. J.; Fialkowski, M.; Smoukov, S. K. Micro- and nanotechnology via reaction− diffusion. Soft Matter. 2005, 1, 114−128. (21) Lagzi, I.; Kowalczyk, B.; Grzybowski, B. A. Liesegang Rings Engineered from Charged Nanoparticles. J. Am. Chem. Soc. 2010, 132, 58−60. (22) Xie, S.; Zhang, X.; Yang, S.; Paau, M. C.; Xiao, D.; Choi, M. M. F. Liesegang rings of dendritic silver crystals emerging from galvanic displacement reaction in a liquid-phase solution. RSC Adv. 2012, 2, 4627−4631. (23) Stone, D. A.; Goldstein, R. E. Tubular Precipitation and Redox Gradients on a Bubbling Template. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 11537−11541. (24) Kapoor, S.; Lawless, D.; Kennepohl, P.; Meisel, D.; Serpone, N. Reduction and Aggregation of Silver Ions in Aqueous Gelatin Solutions. Langmuir 1994, 10, 3018−3022. (25) Harada, M.; Katagiri, E. Mechanism of Silver Particle Formation during Photoreduction Using In Situ Time-Resolved SAXA Analysis. Langmuir 2010, 26, 17896−17905.

precipitation patterns of Liesegang systems reported so far were obtained (Figure 4). From these results, we can conclude that

Figure 4. Optical microscope images of gelatin after contact with a periodic array of agarose stamps at (a) vertexes and (b) sides of the patterns. The scale bars correspond to 5 mm.

the current system with its novel Liesegang mechanism has great potential as a robust approach to producing chemically programmed micropatterned materials whose sizes, shapes, symmetry, spacing, and especially materials can be largely designed by modifying the experimental conditions.

■

CONCLUSIONS We demonstrated Liesegang pattern formation via the chemical reduction of metal ions with no concentration threshold. The key feature is the involvement of a complex-formation process before nucleation and aggregation. Although the concentration threshold is provided by an Ag-methionine complex in the present model system, both or either Ag or methionine could be freely replaced. When appropriate choices of substrate X and complexation site Y are made so that the X-Y complex can be formed, a variety of desired Liesegang patterns consisting of product Z of chemical reaction X + Y → Z can be yielded, where Z could be an inorganic salt, metal nanoparticles, organic nanocrystals, polymeric fibers, or other material. Further investigation will yield such novel Liesegang patterns and also offer a scaffold for novel models of not only chemical but also biological, petrological, and economic reaction-diffusion phenomena.

■

ASSOCIATED CONTENT

* Supporting Information S

Pattern formation in a one-dimensional system, in situ optical absorption measurements, and pattern formation on agarose instead of gelatin. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Liesegang, R. E. Uber Einige Eigenschaften von Gallerten. Naturwiss. Wochenschr. 1896, 11, 353−362. (2) Sultan, R.; Sadek, S. Patterning Trends and Chaotic Behavior in Co2+/NH4OH Liesegang Systems. J. Phys. Chem. B 1996, 100, 16912− 16920. (3) Volford, A.; Izsák, F.; Ripszám, M.; Lagzi, L. Pattern Formation and Self-Organization in a Simple Precipitation System. Langmuir 2007, 23, 961−964. 5050

dx.doi.org/10.1021/la5003786 | Langmuir 2014, 30, 5047−5051

Langmuir

Letter

(26) Lagzi, I.; Ueyama, D. Pattern Transition Between Periodic Liesegang Pattern and Crystal Growth Regime in Reaction−Diffusion Systems. Chem. Phys. Lett. 2009, 468, 188−192. (27) Canamares, M. V.; Gracia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Comparative Study of the Mophology, Aggregation, Adherence to Glass, and Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles Prepared by Chemical Reduction of Ag+ Using Citrate and Hydroxylamine. Langmuir 2005, 21, 8546−8553. (28) Izsák, F.; Lagzi, I. Simulation of a Crossover from the Precipitation Wave to Moving Liesegang Pattern Formation. J. Phys. Chem. A 2005, 109, 730−733. (29) Ostwald, W. Lehrbuch der Allgemeinen Chemie; Engelmann: Leipzig, 1891.

5051

dx.doi.org/10.1021/la5003786 | Langmuir 2014, 30, 5047−5051